Steve is an octopus-human hybrid sort of creature who gets captured by Hydra while they still have Bucky under their control. They send the Winter Soldier into Steve's cell with a bullwhip in hopes of literally whipping Steve into submission.
However, Steve mistakes the whip for a tentacle and Bucky for a female. While he pities 'the poor female' for only having one tentacle, he's still impressed, decides they should be mates, and goes about wooing 'her'.
Found! Whip Crack, by Quarra: https://archiveofourown.org/works/10057091?view_full_work=true